Prior to 1804 von Bila used the mirliton but was supposed to change over the tchako in 1804 – from what I have seen the headwear in the Prussian hussar regiments in 1806 was quite varied, but Doepler is quite right in that they were supposed to be wearing the tchako

Conjecture is your best friend. Fortunately there were no cameras in the Napoleonic wars so you can pretty much go with either. But if I had to go with one I would suggest Doepler would be more trusted.

Doepler is an excellent source as close as possible to the Prussian Army of 1806 – depicted in wonderful detail, some plates can be seen in the ASK Brown collection, in case of doubt -I would go with Doepler.

An article about Prussian Hussars 1806 / 07 published by Herbert Knötel in Zeitschrift für Heeres- und Uniformkunde 1959, Nr. 162 – he uses a French source – in which for Bila Husaren – the shako is given.

I'm glad about what you write about Doepler. I had bought the great (and large) volume "The Uniforms of the Prussian Army 1805/1806" which contains reproductions of all his marvellous plates from the "Stiftung Schlösser und Gärten Berlin-Brandenburg" in large format.